Programming/BASIC
The Run/Stop key is handled by the BASIC STOP vector, which is located at $0326/$0327 (806/807). The default value of the vector is $F265.
Because BASIC execution is too slow to change both bytes of this vector, normally only the low byte ($0326/806) is changed. By analyzing all BASIC programs in our database, we found the following usage:
| POKE value | Number of programs | Example program(s) | Effects | | 24 | 1 | Quarterback | Works, noticeable slowdown in execution (1) | | 32 | 2 | Logi-Toli, Amiga Emu | Works, noticeable slowdown in execution (1) | | 35 | 2 | Blokovnica, Cokin Super | Works, noticeable slowdown in execution (1) | | 101 | 7 | Multi-disk Hungarian | Enable only (restores the default value) | | 103 | 57 | Fantil, Push Back, Varia | Works, unsafe (2) | | 115 | 6 | Popesys, Directory Filer | Works, SOUND statement hangs (3) | | 132 | 3 | Pojmaj Slovo!, Európa (OKTA) | Works, SOUND statement hangs (3) | | 136 | 3 | A Hős Lovag, A Bűvös Gyűrű | Works, SOUND statement hangs (3) | | 237 | 1 | Primus | Odd side effects, SOUND statement hangs (3) |
The ROM was not designed with this in mind; we're changing the $F265 address to another "random" address (from $F200-$F2FF, since only the low byte is changed), but none of them are fully suitable. Here is the breakdown of the side effects.
(Group 1) 24 / 32 / 35: these values work, but the BASIC program will run noticeably slower. The reason for this is that the vector is redirected to extra useless JSR statements. Not recommended; disabling the Run/Stop key is not worth the worsened performance.
(Group 3) 115 / 132 / 136 / 237: these values do not work if your program uses the SOUND statement. Example:
0 POKE806,115
1 VOL4:F=F+1:SOUND1,F,1:GOTO0
The program will hang as soon as it reaches the SOUND statement.
103: From the table, we can see that this is the most common method used by far. It does mostly work, however, it is "unsafe". The default code at $F265 starts with LDA $91 / CMP #$7F, which checks the location of $91 and compares it with $7F (127) to see if Run/Stop is pressed. This POKE points the vector to $F267, which "skips" the LDA statement. Therefore, the CMP statement will still execute, but the value that it compares it with is "undetermined" (whatever happened to be in the A register). If the rest of the code uses POKE or PEEK, and the value POKEd or PEEKed is 127, the program will stop. Here's an example:
0 POKE806,113
1 POKE3072,127:GOTO0
The program will stop in LINE 1. Note that if we omit the Run/Stop disabling POKE, it works fine.
You may be asking: can we "skip" the entire vector by pointing it to an RTS opcode, so that it does "nothing"? Unfortunately, this doesn't work at all. POKE 806, 114 would achieve this, but this causes the BASIC program not to run at all. The problem is, this vector must return with the zero flag not set.
What is a proper (doesn't slow down the code, allows SOUND statements, doesn't break for PEEK/POKEs)? One possible method is this: we POKE a short routine to any free memory area with the low byte being $65 - therefore only the high byte of the vector needs to be changed. Writing LDA #$01 / RTS to $0365 would work:
POKE 869, 169 : POKE 870, 1 : POKE 871, 96 : POKE 807, 3
A similar method has been used in the following programs:
Pojmaj Slovo!, Európa (OKTA), Otritsanie (earliest is 1986, by László Agócs) using 7 POKEs.
Castle Matcher, Felcsút Tycoon, The Librarian (earliest is 2017, by RoePipi) using only a single POKE. The ML part cleverly hidden in the BASIC code and falls exactly to $1065.
Another alternative:
POKE 807, 228 : POKE 806, 13
This will redirect the vector to $E40D in two steps. The order matters; the high byte must be changed first.
Use POKE 786,68 for the same effect. You can enable it with POKE 786,66. This also hangs the SOUND statement.
Another method is using the combination of TRAP and RESUME NEXT BASIC commands.
For example:
10 TRAP 100
20 PRINT "HELLO"
30 GOTO 20
100 RESUME NEXT
However, the program can be stopped by pressing the Run/Stop key multiple times within short period of time.
disable run stop,disable run/stop,disable runstop, |
